docs(examples): translate run_demo.py comments to English and align with package imports

[MarcoPorcellato]

Context

examples/run_demo.py has Italian comments and uses sys.path manipulation instead of package imports after uv sync.

Objectives

• Translate comments and error messages to English
• Use from logseq_matryca_parser... imports without sys.path hack when possible
• make check passes on examples/run_demo.py

Files

• examples/run_demo.py

Getting started

1. uv sync --all-extras
2. uv run python examples/run_demo.py

Out of scope

• New example features

Definition of Done

• make all passes
• Optional CHANGELOG.md docs bullet